The invention relates to a planar antenna according to the features of the preamble of claim 1.
Planar antennas are known in the art. On top of their inherent interesting characteristics of size, cost and radiation performances, these antennas are generally used in all possible cases when an antenna radiation pattern change is required. Often a switching between two distinct configurations of the planar antenna is required. If used as detection antenna, these configurations correspond to two detection or radiation pattern widths. In order to provide different detection or radiation patterns, it is well known in the art to use two or more antennas. Other techniques consist of changing the detection or radiation pattern of the antenna by the use of absorbing material or metallic surrounding configurations. However, all of these techniques are based on a mechanical modifications of the antenna.
U.S. Pat. No. 6,175,723 relates to a self structuring antenna system which comprises a switchable antenna array as planar antenna. The antenna array is defined by a plurality of antenna elements that are selectively electrically connectable to each other by a series of switches. This allows the alteration of the physical shape of the antenna array without actually moving or mechanically changing the antenna. By changing the physical shape, the radiation pattern or lobe of the antenna can be altered. Furthermore, the shape of the antenna can be adapted to changing electrical and/or physical environments in order to achieve a good antenna performance. However, the switches are relays, either solid-state, mechanical or opto-electronic, generally mounted behind the antenna itself. Such switches are expensive and difficult to process. Therefore, this antenna structure is too expensive for cost-sensitive applications.
A typical cost-sensitive application for a planar antenna is an automatic door opener. Automatic door openers use door opener sensors which are usually made of waveguide transceivers. The output of the waveguide transceiver is a waveguide flange. However, it is very difficult and expensive to design a switch for different antennas which can be used in waveguide technology.
Therefore, it is an object of the invention to provide a planar antenna which can be produced at low costs and, therefore, can be used advantageously in low-cost applications, particularly in door opener sensors.
The planar antenna according to the invention is characterized by what is specified in the independent claim 1.
Advantageous embodiments of the invention are specified in the dependent claims.
A further object of the invention is to use such a planar antenna as part of a device performing a parallel traffic rejection algorithm which processes the information received from the planar antenna in such a way that at least two different lobes of the planar antenna are analyzed in order to calculate the direction of a pedestrian moving in or near to the area covered by a Door opener sensor
An inventive planar antenna comprises the following elements:
a plurality of antenna elements positioned relative to each other in a predetermined orientation; each of the plurality of antenna elements being selectively electrically connectable to one or more of the other antenna elements;
a plurality of switches which electrically connect the plurality of antenna elements so that closing one of the switches causes at least two antenna elements to be electrically connected,
an antenna array which is defined by the plurality of switches in combination with the plurality of antenna elements;
the antenna elements are positioned on a planar substrate in such a way that at least two different lobes of the antenna can be provided by activating different antenna elements by means of the switches located on the antenna substrate itself.
According to the invention a planar antenna has been designed which can be used in direct coupling to a waveguide output. This antenna is made of a substrate, particularly a microwave substrate, which comprises antenna elements. The antenna elements can comprise patches on the substrate. In a preferred embodiment the patches are etched on the microwave substrate. According to the invention, it is preferred to generate a switching function on the planar substrate itself than in the waveguide. The inventive antenna can be produced at low costs, particularly using microelectronic production techniques which are widely used and cheap.
The patches can comprise a central patch which performs a coupling function to a microwave circuitry, such as a waveguide, a coaxial probe, a hole or a slot coupling. Other types of microwave circuits with a connection via hole, coaxial probe, etc. are useable. All other patches should have a length which is optimized to make the respective patch resonate at a central frequency and a width which is adjusted to the impedance and radiation power of the antenna.
Preferably, a waveguide is provided for coupling to the planar antenna. The waveguide can comprise a transition to the planar antenna which is terminated by a waveguide flange. In a preferred embodiment, the transition comprises a xe2x80x9cDoggy bonexe2x80x9d filter which reduces spurious radiation at harmonic frequencies. The distance between the xe2x80x9cDoggy bonexe2x80x9d filter and the plane of the planar antenna should be at least about half a waveguide length when the waveguide is mounted on the surface of the planar antenna to achieve good filtering performance. In order to ensure a constant electromagnetic field on its rear aperture and to provide impedance matching, the waveguide can be enlarged in its larger dimension. Finally, the waveguide can comprise a rectangular aperture which is designed to provide enough energy to the central patch and to ensure a good matching between the waveguide and the planar antenna.
In order to avoid microwave leakage through the antenna control lines and ensure a total isolation of the planar antenna from the rest of the antenna control circuitry, a part of the control lines and the connection pads for applying DC current to the switches should be covered by a material absorbing microwaves.
According to the invention, PIN diodes are preferred as switches. The feeding of the PIN diodes and their insertion inside the antenna layout requires taking into account that these extra components should not modify or perturb the radiation characteristics of the antenna. According to the invention, the path used by the DC current to polarize the PIN diodes has been designed to have no influence on the antenna radiation pattern. In this context, the location of the connection of the line for feeding a PIN diode on the patch on which the PIN diode is mounted is of primary importance.
In a further aspect, the invention provides a circuitry for controlling the planar antenna by obtaining at least one Doppler Signal Sample from at least one measurement device working with at least one lobe of the planar antenna, processing the obtained Doppler Signals according to an algorithm and performing a high-speed switching between the configurations of the planar antenna in accordance to the algorithm.
The circuitry preferably comprises sample and hold circuits for sampling the obtained Doppler Signals. The sample and hold circuits can be synchronized with the high-speed switching. Advantageously, the circuitry can comprise a digital signal processor for processing the at least one Doppler Signal. In a preferred embodiment, the digital signal processor processes two Doppler signals obtained from two measurement devices, preferably Schottky diodes positioned in a transceiver of the antenna, and corresponding to two different lobes of the planar antenna and calculates from the Doppler signals an intermediate lobe by weighting the Doppler signals. The circuitry can also comprise an oscillator which produces a sampling frequency signal with an accuracy suitable for sampling. In cases of the use of a high stability frequency source, the circuitry can be formed to perform weighting of the two lobes by pulse width modulation of the antenna control signal. The weighting between the two antenna lobes would be achieved by the meaning action of the amplifier low pass filter. Then, at least one detection chain would be sufficient to retrieve the weighted Doppler signal.
The preferred field of application of the planar antenna are Door opener applications. Advantageously the antenna is used in a Door opener sensor.
Such a Door opener sensor can perform a parallel traffic rejection algorithm which processes the information received from the planar antenna in such a way that at least two different lobes of the planar antenna are analyzed in order to calculate the direction of a pedestrian moving in or near to the area covered by the Door opener sensor. In contrast to known Door opener sensors, traffic in front of a door can be detected more securely in that a door controlled by the sensor only opens when a person intends to enter the door, i.e., the person walks in a certain direction which falls within a detection area of the sensor.